Date:   Tue, 12 Sep 2023 08:08:46 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        maple-tree@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org,
        Shanker Donthineni <sdonthineni@...dia.com>
Subject: Re: [PATCH v2 1/2] maple_tree: Disable mas_wr_append() when other
 readers are possible

On Tue, Sep 12, 2023 at 10:29:30AM -0400, Liam R. Howlett wrote:
> * Liam R. Howlett <Liam.Howlett@...cle.com> [230912 09:56]:
> > * Paul E. McKenney <paulmck@...nel.org> [230912 06:00]:
> > > On Tue, Sep 12, 2023 at 10:34:44AM +0200, Geert Uytterhoeven wrote:
> > > > Hi Paul,
> > > > 
> > > > On Tue, Sep 12, 2023 at 10:30 AM Paul E. McKenney <paulmck@...nel.org> wrote:
> > > > > On Tue, Sep 12, 2023 at 10:23:37AM +0200, Geert Uytterhoeven wrote:
> > > > > > On Tue, Sep 12, 2023 at 10:14 AM Paul E. McKenney <paulmck@...nel.org> wrote:
> > > > > > > On Mon, Sep 11, 2023 at 07:54:52PM -0400, Liam R. Howlett wrote:
> > > > > > > > * Paul E. McKenney <paulmck@...nel.org> [230906 14:03]:
> > > > > > > > > On Wed, Sep 06, 2023 at 01:29:54PM -0400, Liam R. Howlett wrote:
> > > > > > > > > > * Paul E. McKenney <paulmck@...nel.org> [230906 13:24]:
> > > > > > > > > > > On Wed, Sep 06, 2023 at 11:23:25AM -0400, Liam R. Howlett wrote:
> > > > > > > > > > > > (Adding Paul & Shanker to Cc list.. please see below for why)
> > > > > > > > > > > >
> > > > > > > > > > > > Apologies on the late response, I was away and have been struggling to
> > > > > > > > > > > > get a working PPC32 test environment.
> > > > > > > > > > > >
> > > > > > > > > > > > * Geert Uytterhoeven <geert@...ux-m68k.org> [230829 12:42]:
> > > > > > > > > > > > >     Hi Liam,
> > > > > > > > > > > > >
> > > > > > > > > > > > > On Fri, 18 Aug 2023, Liam R. Howlett wrote:
> > > > > > > > > > > > > > The current implementation of append may cause duplicate data and/or
> > > > > > > > > > > > > > incorrect ranges to be returned to a reader during an update.  Although
> > > > > > > > > > > > > > this has not been reported or seen, disable the append write operation
> > > > > > > > > > > > > > while the tree is in rcu mode out of an abundance of caution.
> > > > > > > > > > > >
> > > > > > > > > > > > ...
> > > > > > > > > > > > > >
> > > > > > > >
> > > > > > > > ...
> > > > > > > >
> > > > > > > > > > > > > RCU-related configs:
> > > > > > > > > > > > >
> > > > > > > > > > > > >     $ grep RCU .config
> > > > > > > > > > > > >     # RCU Subsystem
> > > > > > > > > > > > >     CONFIG_TINY_RCU=y
> > > > > > >
> > > > > > > I must have been asleep last time I looked at this.  I was looking at
> > > > > > > Tree RCU.  Please accept my apologies for my lapse.  :-/
> > > > > > >
> > > > > > > However, Tiny RCU's call_rcu() also avoids enabling IRQs, so I would
> > > > > > > have said the same thing, albeit after looking at a lot less RCU code.
> > > > > > >
> > > > > > > TL;DR:
> > > > > > >
> > > > > > > 1.      Try making the __setup_irq() function's call to mutex_lock()
> > > > > > >         instead be as follows:
> > > > > > >
> > > > > > >         if (!mutex_trylock(&desc->request_mutex))
> > > > > > >                 mutex_lock(&desc->request_mutex);
> > > > > > >
> > > > > > >         This might fail if __setup_irq() has other dependencies on a
> > > > > > >         fully operational scheduler.
> 
> This changes where the interrupts become enabled, but doesn't stop it
> from happening.  It still throws a WARN after init_IRQ(). I suspect it
> is not the way to proceed as there are probably many places that will
> need to be changed in both common and arch specific code.  As soon as
> TIF_NEED_RESCHED is set, then all the checks will need to be altered.

Thank you for trying it!

> I think we either need to set the boot thread to !idle, avoid call_rcu()
> to set TIF_NEED_RESCHED (how was this working before?  Do we need rcu
> for the IRQs?), or alter the boot order (note this is NOT arch or
> platform code here).
> 
> I don't like any of these.  I'd like another option, please?

My favorite is to move the interrupt enabling later, but Michael Ellerman
would know better than would I about the feasibility of this.

							Thanx, Paul

> > > > > > >
> > > > > > > 2.      Move that ppc32 call to __setup_irq() much later, most definitely
> > > > > > >         after interrupts have been enabled and the scheduler is fully
> > > > > > >         operational.  Invoking mutex_lock() before that time is not a
> > > > > > >         good idea.  ;-)
> > > > > >
> > > > > > There is no call to __setup_irq() from arch/powerpc/?
> > > > >
> > > > > Glad it is not just me, given that I didn't see a direct call, either.  So
> > > > > later in this email, I asked Liam to put a WARN_ON_ONCE(irqs_disabled())
> > > > > just before that mutex_lock() in __setup_irq().
> 
> Oh, and also:
> arch/powerpc/platforms/powermac/setup.c:        .init_IRQ               = pmac_pic_init,
> 
> > 
> > I had already found that this is the mutex lock that is enabling them.
> > I surrounded the mutex lock to ensure it was not enabled before, but was
> > after.  Here is the findings:
> > 
> > kernel/irq/manage.c:1587 __setup_irq:
> > [    0.000000] [c0e65ec0] [c00e9b00] __setup_irq+0x6c4/0x840 (unreliable)
> > [    0.000000] [c0e65ef0] [c00e9d74] request_threaded_irq+0xf8/0x1f4
> > [    0.000000] [c0e65f20] [c0c27168] pmac_pic_init+0x204/0x5f8
> > [    0.000000] [c0e65f80] [c0c1f544] init_IRQ+0xac/0x12c
> > [    0.000000] [c0e65fa0] [c0c1cad0] start_kernel+0x544/0x6d4
> > 
> > Note your line number will be slightly different due to my debug.  This
> > is the WARN _after_ the mutex lock.
> > 
> > > > >
> > > > > Either way, invoking mutex_lock() early in boot before interrupts have
> > > > > been enabled is a bad idea.  ;-)
> > > > 
> > > > I'll add that WARN_ON_ONCE() too, and will report back later today...
> > > 
> > > Thank you, looking forward to hearing the outcome!
> > > 
> > > > > > Note that there are (possibly different) issues seen on ppc32 and on arm32
> > > > > > (Renesas RZ/A in particular, but not on other Renesas ARM systems).
> > > > > >
> > > > > > I saw an issue on arm32 with cfeb6ae8bcb96ccf, but not with cfeb6ae8bcb96ccf^.
> > > > > > Other people saw an issue on ppc32 with both cfeb6ae8bcb96ccf and
> > > > > > cfeb6ae8bcb96ccf^.
> > > > >
> > > > > I look forward to hearing what is the issue in both cases.
> > > > 
> > > > For RZ/A, my problem report is
> > > > https://lore.kernel.org/all/3f86d58e-7f36-c6b4-c43a-2a7bcffd3bd@linux-m68k.org/
> > > 
> > > Thank you, Geert!
> > > 
> > > Huh.  Is that patch you reverted causing Maple Tree or related code
> > > to attempt to acquire mutexes in early boot before interrupts have
> > > been enabled?
> > > 
> > > If that added WARN_ON_ONCE() doesn't trigger early, another approach
> > > would be to put it at the beginning of mutex_lock().  Or for that matter
> > > at the beginning of might_sleep().
> > 
> > Yeah, I put many WARN() calls through the code as well as tracking down
> > where TIF_NEED_RESCHED was set; the tiny.c call_rcu().
> > 
> > 
> > So my findings summarized:
> > 
> > 1. My change to the maple tree makes call_rcu() more likely on early boot.
> > 2. The initial thread setup is always set to idle state
> > 3. call_rcu() tiny sets TIF_NEED_RESCHED since is_idle_task(current)
> > 4. init_IRQ() takes a mutex lock which will enable the interrupts since
> > TIF_NEED_RESCHED is set.
> > 
> > I don't know which of these things is "wrong".
> > 
> > I also looked into the mtmsr register but decided to consult you lot
> > about my findings in hopes that someone with more knowledge of the
> > platform or early boot would alleviate the pain so that I could context
> > switch or sleep :)  I mean, an mtmsr bug seems like a leap even for the
> > issues I create..
> > 
> > Regards,
> > Liam
> > 
> > 
> > -- 
> > maple-tree mailing list
> > maple-tree@...ts.infradead.org
> > https://lists.infradead.org/mailman/listinfo/maple-tree

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